The present invention relates to three-dimensional (3D) computerized tomography (CT) and, more particularly, methods and systems providing an appropriate step size in a complete data scanning path for cone beam CT.
In conventional computerized tomography for both medical and industrial applications, an x-ray fan beam and a linear array detector are employed. Two-dimensional (2D) imaging is achieved. While the data set is complete and image quality is correspondingly high, only a single slice of an object is imaged at a time. When a 3D image is required, a stack of slices approach is employed. Acquiring a 3D data set one 2D slice at a time is inherently slow. Moreover, in medical applications, motion artifacts occur because adjacent slices are not imaged simultaneously. Also, dose utilization is less than optimal, because the distance between slices is typically less than the x-ray collimator aperture, resulting in a double exposure to many part of the body.
In 2D CT, the scanning path of the source is often a simple circular scan about the object. The linear array detector is fixed relative to the source. (Although it is usual to talk about a scan path of a source relative to the object to be imaged, it is to be appreciated that the object may be rotated or otherwise moved to provide relative motion between the object and the source). The data from the linear array detector is acquired at uniform angular intervals .DELTA..THETA.. For a system with a center-to-center separation between adjacent detectors of .DELTA.w, a field of view radius r and magnification M, .DELTA..THETA. is typically chosen to be EQU .DELTA..THETA.=(.DELTA.w/M)/r
This choice simply corresponds to the fact that for data acquired at two adjacent view angles spaced by .DELTA..THETA. to be independent, a point on the circumference of the object (distance r from the center) must move from in front of one detector element to in front of the
adjacent detector element ##EQU1## between view angles.
In a system employing true cone beam geometry fro 3D imaging, a cone beam x-ray source and a 2D area detector are used. An object is scanned, preferably over a 360.degree. angular range, either be moving the x-ray source in a scanning circle around the object or by rotating the object while the source remains stationary. In either case, the area detector is fixed relative to the source. The relative movement between the source and object which is to be imaged provides scanning in either case. Compared to the conventional 2D stack of slices approach to achieve 3D imaging, the cone beam geometry has the potential to achieve rapid 3D imaging of both medical and industrial objects, with improved dose utilization.
The standard scanning path used in cone beam 3D CT imaging is a single circle scan of source and detector around the object. The detector is typically sampled at uniform angular intervals around the circle as in 2D CT. However, the data acquired in a single scanning circle can be shown to be incomplete for 3D CT imaging.
Complete data scanning paths are known, but the paths are not confined to a single plane making the proper choice of a sample interval significantly different from that in the 2D or 3D circular scan case. U.S. Pat. No. 5,073,910 issued Dec. 17, 1991 to the present inventor and Hedengren discloses a complete data scanning path. The patent, which is assigned to the present assignee, is hereby incorporated by reference.
The criteria for data set completeness relative to scanning path in a 3D CT system are described in the paper by Bruce D. Smith entitled "Image Reconstruction From Cone-Beam Projections: Necessary and Sufficient Conditions and Reconstruction Methods", IEEE Transactions on Medical Imaging, Volume MI-4, No. 1, pages 14-25 (March 1985), hereby incorporated by reference.
Whether or not a non-planar scan path for cone beam 3D CT is complete path, locations in the scan path must be identified for acquiring data by the area detector. The source may be moved continuously with data being acquired from the area detector at different locations. Generally, the speed of data acquisition is relatively high compared to the movement of the source such that the small amount of movement of the source during the data acquisition will not introduce unacceptable effects. The data acquisition may be thought of as somewhat similar to use of high speed film to take a photograph of a moving object. If the film and shutter speed are sufficiently fast relative to the movement of the object, the film will show the object to be essentially stationary.
As an alternative to continuous motion scanning, one could use step wise scanning. In step wise scanning, one would move the source relative to the object being imaged, acquire data from the area detector while there is no relative motion between the object and the source, and move the source relative to the object after data acquisition is complete for a given location. The source is then stopped at a second location for data acquisition, data is then acquired, and the source is moved on to a third location for data acquisition and so forth.
Whether the relative motion of the source to the object occurs in a continuous fashion or the motion occurs in a step wise fashion as described, one should determine the locations in the trajectory at which data should be acquired. If the intervals .DELTA.s in the trajectory s are relatively small, one can obtain the most accurate image. However, making the intervals .DELTA.s relatively small will result in obtaining a large amount of redundant data. That is, the data from one data acquisition point in the trajectory s will have a significant overlap with the data from the area detector at the adjacent data acquisition point in the trajectory. More importantly, using especially small intervals .DELTA.s requires a relatively large amount of computer processing power. That is, real-time processing of data becomes more difficult as the stream of data increases from use of smaller intervals .DELTA.s in the trajectory s. Furthermore, it is hard to justify the extra cost to obtain the necessary computer processing power to process data corresponding to relatively small intervals .DELTA.s if the data has a relatively high level of redundancy.
In either the continuous motion or step wise motion scan technique, one can reduce the requirements for data processing power by increasing the size of the intervals .DELTA.s separating adjacent data acquisition points in the trajectory s. However, if the intervals .DELTA.s between data acquisition points in the trajectory s are too large, independent or nonredundant data will be lost and artifacts and/or distortions will be introduced into the image data.